A base station device is an important component of a cellular mobile communications system and refers to a radio transceiver station that transfers information with a mobile phone terminal through a mobile communication switching center within certain radio coverage. A base station device generally includes a receiving device and a transmitting device; where the receiving device and the transmitting device are collectively called a transceiver.
For a frequency-division duplex transceiver, to ensure that a receiving device and a transmitting device can work at the same time, a duplexer is usually used to isolate a transmitted signal and a received signal. The duplexer includes two groups of filters of different frequencies to avoid transmitting a transmitted signal of the transceiver to a receiving device, and to suppress the interference and noise entering the receiving device at the same time, thereby suppressing outward spurious emission and interference of the transmitting device.
A multi-mode and multi-carrier base station transceiver may support a plurality of communications standards, such as GSM (Global System of Mobile Communication) and WCDMA (Wideband Code Division Multiple Access). A base station device that supports a plurality of carriers facilitates network expansion. Therefore, more and more operators use a multi-carrier and multi-mode base station.
If a transmitted signal of the duplexer is not suppressed appropriately for the receiver, another filter following the receiver, such as a SAW (Surface Acoustic Wave), may be used for further filtering; however, when intermodulation of the transmitted signals of a multi-carrier base station falls into a receive frequency band through the duplexer, if the intermodulation shares a frequency with a received signal, the intermodulation will directly interfere with the received signal. For example, assuming that, in a 1800 MHz GSM frequency band, two transmitted signals are 1805 MHz and 1828.75 MHz and a gap between the two signals is 23.75 MHz, 7-order intermodulation of the transmitted signals is 1805 MHz−3*23.75 MHz=1733.75 MHz, and received signals happen to be 1710 MHz and 1733.75 MHz, in this case, a receive filter does not filter for the frequency band and therefore only a transmit filter of the duplexer can be used for suppressing. An example is taken where each carrier of a transmitted signal has transmit power of 40 dBm, transmitter remote intermodulation (that is, a power difference between intermodulation and a carrier) is 60 dBc, a signal resolution bandwidth is 200 kHz, and noise of an interference signal that enters a receiver is required 10 dB lower than thermal noise at an antenna port, a receive-transmit suppression degree of a transmit filter in a receive frequency band should be (40-60) dBm−(−174 dBm+10*lg(200 k)−10)=111 dB, where −174 dBM is thermal noise per Hz in normal temperature at the antenna port. A duplexer can achieve such a high suppression degree only by adding filter cavities, which leads to a large size of the duplexer.
With an increasing demand for miniaturization of communications base station devices and emergence of new-form base stations (such as a base station with an active antenna), it is quite important to reduce difficulty in duplexer design and reduce a suppression degree of a duplexer.